Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
As used herein, zeolites may be referred to by improper name, such as beta, proper name, such as CIT-6, or by structure type code, such as *BEA. These three letter codes indicate atomic connectivity and hence pore size, shape and connectivity for the various known zeolites. The list of these codes may be found in the Atlas of Zeolite Framework Types, which is maintained by the International Zeolite Association Structure Commission at http://www.iza-structure.org/databases/. At present, 234 structure types are known and catalogued by the IZA. One such structure type, *BEA has been described in the literature and is known to contain 3-dimensional 12-ring channels. Zeolites are distinguished from each other on the basis of their composition, crystal structure, and adsorption properties. One method commonly used in the art to distinguish zeolites is x-ray diffraction. A newer x-ray method is small angle x-ray scattering, SAXS.
Beta zeolite was first described structurally in the literature by Higgins, et al. in Zeolites, 1988, 8, 446-52. Using tetraethylammonium as the cationic organic structure directing agent (OSDA), they crystallized a material having a powder x-ray diffraction pattern containing a combination of sharp and broad reflections at SiO2/Al2O3 ratios in the range of 30-50. They identified disorder along the [001] direction and proposed three polymorphs of the beta zeolite, A, B, and C. These three polymorphs are constructed from the same layer but with different shifts along the a or b axes. Beta zeolite has at least peaks in the x-ray diffraction pattern at 7.5, 13.44, 21.38, and 22.43°2θ using Cu—Kα radiation.
Corma and coworkers (Chem. Mater. 2008, 20, 3218-23) synthesized materials significantly enriched in polymorph B using 4,4-dimethyl-4-azoniatricyclo[5.2.2.0]undec-8-ene as the OSDA. This material has an x-ray diffraction pattern with peaks at positions of at least 7.5, 8.2, 13.5, 21.4, and 22.4°2θ using Cu—Kα radiation.
Polymorph C has been synthesized by Moliner, et al., using 4,4-dimethyl-4-azoniatricyclo[5.2.2.0]undec-8-ene as the OSDA. This polymorph has been given the structure code BEC. The ITQ-17 material so produced (J. Phys. Chem. C 2008, 112, 19547-54) comprises titanium and has an x-ray diffraction pattern with peaks at positions of at least 6.9, 9.6, 15.3, 19.2, and 22.2°2θ using Cu—Kα radiation.
In addition to the A, B, and C polymorphs, two additional polymorphs of the beta zeolite system have been synthesized and named SU-78A and SU-78B. Yu, et al., described the SU-78 material synthesized using either N,N-dimethyl-dicyclohexylammonium or N-ethyl-N-methyl-dicyclohexylammonium as the OSDA in Chem. Mater. 2012, 24, 3701-6 as an intergrowth with complex twinning and disorder of the SU-78A and SU-78B polymorphs. These materials comprise germanium and have an x-ray diffraction pattern with at least 9 peaks using Cu—Kα radiation.
Takewaki, et al., discovered the CIT-6 family of materials (J. Phys. Chem. B 1999, 103, 2674-2679) as a composition comprising zinc which can be extracted in various post-synthesis treatments. These materials are of the disordered *BEA type and have an x-ray diffraction pattern with at least 6 peaks using Cu—Kα radiation.
A naturally occurring mineral version of beta zeolite is also known, called tschernichite. Tschernichite usually comprises calcium and has SiO2/Al2O3 ratios in the range of 3-8 and a powder x-ray diffraction pattern containing a combination of sharp and broad reflections. The x-ray diffraction pattern comprises peaks at d-spacings of at least approximately 11.6-12, 6.3, 4.21, 4.02, 3.56, and 3.15 Å using Cu—Kα radiation.
Zeolite beta is of the disordered *BEA zeotype and known to be heavily faulted. A typical preparation comprises about 60% polymorph A and 40% polymorph B with typical ratios of the polymorphs varying by 10-20% according to preparation (Cantin, et al., Angew. Chem. Int. Ed. 2006, 45, 8013-15). Materials comprising intergrowths of the A and B polymorphs are included in the partially disordered material code *BEA. Szostak and coworkers show in J. Phys. Chem. 1995, 99, 2104-9 that tschernichite is also an intergrowth of the A and B polymorphs at about the same ratios.
Many materials described in the literature as nanocrystalline beta zeolites are also known. These include materials synthesized by Mihailova, et al., (Phys. Chem. Phys. Chem. 2005, 7, 2756-63), Ding and Zheng (Mater. Res. Bull. 2007, 42, 584-90), Casci and coworkers (NU-2, Stud. Surf. Sci. Catal. 1989, 49A, 151-160), Perez-Pariente, et al., (Appl. Catal. 1987, 31, 35-64), Kobler, et al., (J. Phys. Chem. C 2008, 112, 14274-80), Larlus, et al., (Micro. Meso. Mater. 2011, 142, 17-25) and Keuchl, et al., (Micro. Meso. Mater. 2010, 127, 104-18). Numerous other zeolites, including other zeolites of the *BEA type, are known.